RU2308781C2 - X-ray tube - Google Patents

Abstract

FIELD: roentgen engineering; pulsed tubes for equipment producing and using roentgen rays in medicine and technology.

SUBSTANCE: proposed X-ray tube has vacuum envelope, anode, and insulator-mounted high-voltage bushing with explosion-emissive cathode; the latter is made of material whose surface nanostructure is self-reproducible upon impact of high-voltage pulses of up to 60 kV in amplitude and 10 - 80 ns in length applied in train at repetition frequency up to 1 MHz. Soft-range radiation obtained in the process ensures high contrast of images when X-ray tube deals with objects of different optical density.

EFFECT: enhanced service life and operating stability; enlarged functional capabilities of tube.

2 cl, 12 dwg

Description

The invention relates to x-ray technology, in particular to pulsed x-ray tubes, and can be used in equipment for obtaining and using x-ray radiation in medicine and technology.

It is known that an x-ray tube is an electrovacuum device for producing x-ray radiation. The main elements of the X-ray tube are the cathode (electron source) and the anode (X-ray source) placed in a vacuum cylinder.

The emission of electrons by a solid surface (cathode) is called electron emission. Electrons cannot spontaneously leave the cathode surface, since for this it is necessary to do work against the internal forces holding them at the cathode-vacuum interface. Thus, in order to release electrons from the cathode, it is necessary to expend energy. By the method by which this energy is transferred to the cathode, emission processes are called thermionic emission, when energy is transferred to electrons when the cathode is heated due to thermal vibrations of the lattice; secondary electron emission, when this energy is transferred by other particles (electrons or ions that bombard the cathode); photoelectron emission, in which electrons are knocked out by light quanta, etc.

Traditional x-ray tubes are based on the use of incandescent cathodes. In these devices, the source of electrons (emitter) is a metal filament, heated to very high temperatures. The need for heating, which is why such electronic emitters are also called thermal cathodes, is due to the fact that at room temperature the electrons have too little energy to emit from the metal.

The prior art x-ray tube containing enclosed in a vacuum flask anode, a control electrode, a window for outputting x-ray radiation and a thermal cathode [1].

The disadvantage of this x-ray tube is that it requires both a high-voltage power source and a source for the cathode to glow. The expenditure of power on the glow of the cathode reduces the efficiency of the tube. In addition, the mandatory heating of the cathode leads to heat generation and undesirable heating of the x-ray tube, therefore, the resource of stable operation of the tube is small.

It is known that electrons can be pulled out of metals even at room temperatures if an electric field sufficient for electron emission is applied. For this reason, the most common name for such electron sources is cold cathodes. Also, such electron sources are called field emission, since the principle of their operation is based on the phenomenon of field emission. In foreign literature, this phenomenon is called field electron emission. Among emission phenomena, field emission occupies a special place, since this is a purely quantum effect, in which the release of electrons from the cathode does not require energy expenditure on the emission act itself, in contrast to thermal, photo, and secondary emission.

Autoelectronic emission is the phenomenon of the emission of electrons into vacuum from the surface of a solid or other medium under the influence of a very strong electric field with a strength of E = 10 ⁷ -10 ⁸ V / cm. In order to create such strong electric fields, tens of millions of volts would have to be applied to ordinary macroscopic electrodes. Practically field emission can be excited at much lower voltages if the cathode is shaped like a thin tip with a tip radius of tenths or hundredths of a micron. Then, even when a relatively moderate electric field is applied, its real value on the micro-tips will be orders of magnitude larger due to geometric amplification. The thinner and longer this tip will be, the greater the amplification effect will be.

Therefore, the traditional direction in the development of field emission emitters is the creation of a field of identical micropoints on the surface of the cathodes.

Known sealed pulsed x-ray tube containing a vacuum housing made of glass, multi-edge field emission cathode and anode located opposite it along the axis of the tube [2].

The disadvantage of this device is the small resource for the stable operation of the tube, since the use of traditional metals as the cathode material leads to a rapid decrease in their emission properties as a result of sputtering and chemical degradation even under high vacuum conditions.

Long-term studies conducted at the Research Center for Electrophysical Problems of the Surface of the Russian Academy of Natural Sciences at St. Petersburg State University of Telecommunications have led to the creation of a new class of portable X-ray machines.

The creation of this new technique is based on the discovery of the phenomenon of explosive electron emission due to the explosive transition of the condensed substance of the cathode into a dense plasma upon heating of the local areas of the cathode by its own emission current.

The essence of the phenomenon lies in the fact that if energy of a high concentration is quickly introduced into a metal cathode, an instantaneous transition of the metal from the solid state to the plasma takes place and a large electron flux is released - this is microexplosion.

Take two metal electrodes in a vacuum. We apply voltage to them and we will increase it. A microammeter connected to the circuit will begin to show current. At the same voltage, it will fluctuate slightly. This is field emission. The metal, even after grinding, is not quite smooth - there are projections and points on it. The field strength is higher on them, and it is from these points that the field emission current is emitted into the vacuum. In this case, the tips can be destroyed, therefore, the field emission current is unstable. If you increase the voltage strongly, the current will increase so much that it will heat the tip until it evaporates. A small cloud of ionized vapor is formed, consisting of atoms, ions and electrons. An electric field pulls out of this cloud part of the electrons that fly to the second electrode. This is the explosive emission that was discovered simultaneously by G.A.Mesyats and G.N. Fursey.

Today it is an important way to obtain powerful and short current pulses in a vacuum. Powerful because the voltage in this process can reach U = 1 MB, current I = 10 kA, and short (1-100 nanoseconds) because there is not much substance in the exploding tip and it is quickly consumed. It is not necessary to dispense with spikes formed on metal accidentally - explosive emission can be obtained from specially made electrodes containing wires or threads.

When braking in the metal of the second electrode, the electrons generate x-rays.

The closest in technical essence and the achieved result to the claimed technical solution is a pulse tube containing a vacuum housing, an anode and an explosion-emission cathode mounted on an insulator [3].

In order to achieve the field necessary for initiating explosive electron emission on the cathode surface (E> 10 ⁷ V / cm), metal foil emitters made in the form of symmetrical sets of blades are used in this x-ray tube.

The disadvantage of this device is the low resource of stable operation of the tube, as well as the fact that such a tube generates a stream of hard x-ray radiation, unsuitable for obtaining a contrast picture when working with objects of different optical density.

It is known that on metals, even in the mode of short (nanosecond) pulses of explosive electron emission current, mass transfer is M = 10 ^-5 g / C. In this case, a significant, unregulated destruction of the surface of the emitters occurs, leading to a violation of the reproducibility of the explosion conditions from pulse to pulse and, as a result, to a limited life of the tubes with such cathodes.

The same reason leads to a decrease in the gain of the external field on the cathode surface - the effect of "polishing", which makes it impossible to use them at low operating voltages, that is, to obtain radiation of a soft range, providing high contrast images.

In addition, vapors and droplets of the cathode material, spraying on the surface of the high-voltage tube insulator, reduce its electric strength, which leads to its surface breakdown from the most vulnerable, vacuum side.

The technical result of the proposed solution is to increase the resource of stable operation of the tube, as well as to expand the functionality of the device by obtaining radiation of a soft range, providing high contrast images when working with objects of different optical density.

To achieve this technical result, in an X-ray tube containing a vacuum housing, an anode and a high-voltage input with an explosion-emission cathode mounted on an insulator, according to the invention, the explosion-emission cathode is made of a material with a nanostructure of the surface, self-reproducing when exposed to high-voltage pulses with an amplitude of up to 60 kV for 10- 80 ns filed by a group with a repetition rate of up to 1 MHz. To achieve this result, the explosion-emission cathode can be made of graphite-based material.

The presence of distinctive features, namely the implementation of an explosive emission cathode from a material with a nanostructure of the surface, self-reproducing when exposed to high-voltage pulses with an amplitude of up to 60 kV for a duration of 10-80 ns, filed by a group with a repetition rate of up to 1 MHz, indicates the compliance of the claimed technical solution with the patentability criterion "novelty".

For a shot-through X-ray tube, we developed an explosive emission cathode with a self-reproducing surface nanostructure, which makes it possible to obtain currents of hundreds of amperes at voltages of tens of kilovolts and a pulse duration of tens of nanoseconds.

It was experimentally established that the implementation of a cathode made of a material such as graphite significantly increases the stable life of the X-ray tube, namely, the number of X-ray pulses without changing its parameters at each pulse, with emission characteristics that are completely reproducible from pulse to pulse (a result of the order of 10 ⁶ )

The occurrence of a liquid phase on a graphite cathode during explosive electron emission is proved, while the process of maintaining explosive electron emission can be represented in the following form.

After the initiation of the explosion and the formation of a dense plasma in some of the most stressed sections of the cathode, the surface transitions from the solid phase (initially the solid-state cathode) to the liquid phase. The current in the diode (x-ray tube) increases due to the expansion of the emitting surface of the outer boundary of the cathode plasma (hereinafter referred to as CP). The density of the CS is high, the layer is very thin, a small potential drop is enough to support emission at inhomogeneities, for which the critical instability field is greater than for field emission.

As the CS expands along the cathode surface, its concentration decreases, the cathode layer increases, and the potential decreases in the cathode layer. However, over time, the area of the melt zone on the cathode surface also increases, which leads to a decrease in the field critical for the excitation of aperiodic instability. Now, the forces of an electric field, acting on a liquid surface, excite an aperiodic instability on it, which leads to the emergence and growth of micropoints. Thus, the maintenance of explosive electron emission occurs due to the continuous regeneration (self-reproduction) of micropoints formed on the liquid surface of the molten cathode in a strong field of the cathode plasma layer, as well as due to their sufficiently large number.

Obviously, the stability of the emission characteristics, and therefore the characteristics of x-ray radiation, depends on the ability to generate and maintain a microstructure from pulse to pulse, namely, many micro points on the cathode surface.

Small-scale inhomogeneities of the surface relief of a graphite cathode were studied using scanning electron microscopy.

When comparing photographs taken with a scanning electron microscope with a maximum spatial resolution of 20 and shown in Figs. 1, 2, traces of melting and movement of molten graphite over the emitter surface are clearly visible.

It turned out that on the “solidified” at room temperature graphite (Fig. 2), significantly thinner micro points were preserved than on metals on which such inhomogeneities could only be preserved when the cathodes were cooled to the temperature of liquid helium.

Micro points were found with a characteristic diameter of about 0.1 μm and a peak curvature of less than 10 nm, uniformly distributed over the surface of the emitter with a density of 10 ⁸ cm ² . Such a microstructure provides high stability and reproducibility of the current pulses of the explosive electron emission of graphite emitters, even at a relatively low voltage on the diode, since the cathode plasma during its explosion efficiently and uniformly forms immediately over the entire surface of the cathode.

Preservation of the microstructure on the surface of the graphite cathode after the termination of the explosive electron emission process is associated with a feature of its solidification mechanism, which is fundamentally different from that for metals.

For an ideal fluid, the characteristic decay time of a perturbation with a width L is

where ρ is the fluid density, α is the surface tension coefficient of the fluid.

For micro points with L = 10 ^-7 -10 ^-5 m, τ _id ~ 10 ^-10 s was established.

For a non-ideal fluid, taking into account viscosity, the characteristic decay time of a perturbation with a width L is

where ν is the kinematic viscosity of the melt, κ = 2π / L is the wave number. A _rough estimate gives τ _neid = 10 ^-9 s.

где r - удельная теплота плавления, κ - коэффициент теплопроводности, что дает τ_заст=10^-9-10^-8 с.Point solidification time due to heat removal to the cathode body

where r is the specific heat of fusion, κ is the coefficient of thermal conductivity, which gives τ _lock = 10 ^-9 -10 ^-8 s.

где l_κ - размер микроострия, ν_пл - скорость разлета плазмы. Для l_κ=10^-7-10^-6 м, полагая ν_пл=10⁴ м/с, получаем τ_разл=10^-11-10^-10 с.The time of plasma expansion above the detonated micropoint can be estimated if we assume that its initial dimensions are comparable with the dimensions of the formed micropoints. It makes up

where l _κ is the size of the micro point, ν _pl is the plasma expansion velocity. For l _κ = 10 ^-7 -10 ^-6 m, setting ν _pl = 10 ⁴ m / s, we obtain τ _dec = 10 ^-11 -10 ^-10 s.

The mechanism of the transition of molten graphite to a solid state is of fundamental importance. Typically, the transition of the liquid phase into the crystalline phase requires the removal of energy approximately equal to the heat of fusion. However, in the case of graphite, this is not a problem, since with a sharp, almost instantaneous pressure drop near the cathode surface, at the end of the explosive electron emission current pulse, graphite transforms into a metastable amorphous glassy form (glass transition) with further energy dissipation.

Thus, the formed microstructure does not decompose under the action of viscous forces, but remains approximately in the form in which it is maintained in the field of the CS layer during explosive electron emission, i.e., it maintains a high external-field gain (β-factor) from pulse to pulse .

Such a microstructure cannot be preserved on the metal - after switching off the external field, the small micro-tips break up, and only rare, smooth, large protrusions randomly distributed over the surface are preserved.

We conducted resource tests of graphite emitters in the mode of explosive electron emission. We studied cathodes made of graphite fabric woven from two mutually intersecting sets of carbon polyacrylonitrile fiber filaments.

To test the cathode, a special generator of rectangular nanosecond pulses of adjustable amplitude and duration was used. It could work without “after pulses” (reflections) at an inconsistent load, with a repetition rate from single pulses to 25 Hz. Output pulses with an adjustable duration of 10 to 75 ns, with a front of 5 ns, were formed with nanosecond synchronization relative to the start due to the discharge of the 50-ohm cable line through sequential three-electrode spark gap according to the scheme of Yu.V. Vvedensky.

Tests have shown high stability of explosive electron emission on graphite cathodes in the voltage range U = 20-80 kV. The stability, reproducibility of the current waveforms is such that they coincide with an accuracy of the line thickness on the oscilloscope screen in the range of current densities at the cathode j≥10 ⁴ A / cm ² (Fig. 3). The resource of stable operation of the cathode was more than 10 ⁵ inclusions.

It is known from the patent information that the cathode is implemented as a carbon fiber bundle in an x-ray tube with an autocathode [4].

However, the known device operates only in field emission mode. In this mode, the cathode current is about I = 10 ^-4 A, which is six orders of magnitude less than in the proposed device.

Together with the implementation of an explosive emission cathode made of graphite, we developed a special mode of high-voltage power supply for an X-ray tube, namely, the supply of high-voltage voltage pulses with a high repetition rate, namely, a group, in a certain sequence, to minimize the time required to set the full dose necessary for reliable fixation, for example, on an X-ray film. So, if N is the number of pulses in the group, and F is the frequency of their repetition, then in our case N = 10 at F = 10 kHz, that is, ten pulses in 0.01 s.

This makes it possible in a short time to obtain the desired dose of x-ray radiation that has passed through the studied object, sufficient for reliable fixation of this object on the radiation receiver. This mode allows you to control the exposure time for objects with different optical depths and provides reliable fixation using the minimum radiation dose.

X-ray hard radiation apparatus (U> 100 kV) is known from patent information, where a burst mode of pulse supply is used to obtain a sufficient exposure dose of radiation [5].

Obviously, in nanosecond soft X-ray sources (U <100 kV) with a duration t of less than 100 ns, the dose per pulse may not be sufficient for reliable recording, for example, in projection radiography problems. This problem can be especially acute with radiography of living objects, that is, in medical devices.

As shown above, the pulse repetition rate of explosive electron emission current with a high stability resource, the reproducibility of the current from pulse to pulse on graphite cathodes is limited only by the decay time of the CS in the diode gap of the tube. From the above estimates it follows that while maintaining a high resource of the number of cathode inclusions, it can be 10 ⁹ Hz. However, to prevent breakdown in the diode gap of the tube when a repeated voltage pulse is applied, it is still necessary to completely expand and recombine both the cathode and the anode plasma arising from the interaction of the electron beam with the target anode of the tube.

Our experiments in the above modes (U <100 kV, t <100 ns, j <10 ⁵ A / cm ² ), with diode gaps D <1.5 cm, showed that the necessary pause between pulses, not leading to breakdown in the diode is 10 ^-6 s. This means that the pulse repetition rate in the group can be F = 1 MHz. Therefore, if it is necessary to obtain a sufficient exposure dose, for example, at N = 100 pulses, for a single voltage pulse of t = 10 ^-7 s, the total exposure time can be T = 10 ^-4 s, which is quite acceptable even for medical radiography.

From the above it follows that the introduced distinguishing features are the reason for obtaining a new result, namely, they allow you to create a powerful pulsed source of soft x-ray radiation with a high resource of stable operation.

The absence of incandescent systems, beam focusing, non-rigid vacuum conditions, the use of pulsed power based on solid-state diodes with an inverse recovery order made it possible to obtain a miniature X-ray tube, as well as significantly reduce the cost of X-ray equipment compared to foreign counterparts (devices from Hewlett Packard, General Electric, MiniXRay and etc.).

The main advantages of the offer are light weight and energy consumption, uniqueness of use, ease of operation, many times longer service life compared to existing analogues, low price. In this case, the equipment is environmentally friendly.

The portability of X-ray equipment and operation from autonomous power sources make it possible to use it in field and extreme conditions.

Since such an X-ray source has a wide range of settings for voltage and pulse repetition rate, it is applicable for solving a variety of problems that require both high resolution in image contrast (analysis of tissues and other optically leaky objects) and sufficient transmission ability (projection defectoscopy )

For the purposes of medical radiography, soft x-ray has the advantage that it has a high contrast sensitivity characterizing the minimum difference in thickness or density of the constituent parts of the object under study, distinguishable by transmission in specific conditions. Such X-ray machines are required in the emergency medical care system and for diagnosis in emergency situations.

From the foregoing it follows that the technical result of the invention is achieved by a new set of essential features, both newly introduced and well-known, therefore, the claimed technical solution meets the patentability criterion of "inventive step".

The proposal is illustrated by drawings, where in Fig.1 shows a photograph of the original surface of a single filament of a graphite cathode before the explosion; figure 2 presents a photograph of the surface of a single filament of a graphite cathode after 10 ⁴ inclusions; figure 3 presents the waveforms of high voltage pulses and x-rays; figure 4 schematically shows an x-ray tube; figure 5 schematically shows the design of a graphite cathode; figure 6 presents a photograph of the surface of a graphite cathode, where individual filaments are visible; figure 7 presents a table with the parameters of the apparatus with the inventive x-ray tube; on Fig presents x-ray introscopy of a microcalculator, voltage U = 37 kV; figure 9 presents an x-ray of a hand with a defect in the joint of the thumb, voltage U = 35 kV; figure 10 presents an x-ray of a biological object - chicken legs, voltage U = 40 kV; figure 11 presents an x-ray of a biological object - the internal organs of the fish, voltage U = 30 kV; on Fig presents an x-ray of a biological object - chicken legs with bone fractures, voltage U = 32 kV.

The x-ray tube (figure 4) contains a vacuum case 1, the anode (target) 2, and an explosive emission graphite cathode 3. A glass high-voltage insulator 4 containing a high-voltage input 5 is fixed to the case 1. The cathode 3 contains a cathode holder with a graphite emitter (figure 5) .

Anode 2 is a combined one, consisting of a tungsten target and a window, which ensures tightness and delays the electrons passing through the target.

The cathode 3 is made of graphite fabric woven from two mutually intersecting sets of carbon polyacrylonitrile fiber filaments. The thread contains several hundred fibers with a diameter of 7-10 microns each. The blank for cathode 3 is a monolithic three-dimensional material formed by many layers of tissue impregnated with pyrocarbon.

The cathode 3 is machined from the billet so that the filaments form, both on the surface and in the bulk, an angle of 45 ° with its longitudinal axis. The emitting surface of the cathode 3 is a two-dimensional matrix of spikes from the ends of the fibers protruding from the monolithic sample at 70-100 μm (Fig.6). The packing density of the fibers is 5 · 10 ³ mm ^-2 .

An X-ray tube was manufactured and tested with the parameters presented in the table in Fig. 7. Examples of x-ray images of various biological objects shown in Figs. 8-12 allow a clear image of objects of different optical densities to be seen.

As a source of high-voltage power, a nanosecond generator was used on semiconductor devices of reverse switching and nanosecond pulse sharpening with an inverse order of recovery, developed at the Physicotechnical Institute named after A.F. Ioffe RAS.

X-ray tube works as follows.

When a voltage pulse with a steep front is applied to the electrodes of the tube, namely, to the anode 2 and cathode 3, an explosive thermal destruction of the micro-tips by field emission current occurs, which are always on the emitters. After the end of the voltage pulse on the surface of the cathode 3, almost instantly, the microstructure solidifies, the repeated application of a sufficient electric field to it leads to the repeated act of explosive electron emission with parameters that fully reproduce the previous emission pulse. As an example, Fig. 9 shows an x-ray of a hand with a defect in the joint of the thumb, obtained by applying 40 pulses of 20 ns duration, voltage U = 35 kV.

From the above it follows that the proposal provides a technical result, does not cause difficulties, involves the use of developed materials and standard equipment, which indicates the compliance of the claimed technical solution with the patentability criterion of "industrial applicability".

Information sources

1. EP patent No. 1037248, IPC H01J 35/02, 09/20/2000.

2. "Non-heated cathodes", ed. M.I. Elinson, M., "Soviet Radio", 1974, S. 260-269.

3. Patent RU No. 2145748, IPC H01J 3/02, H01J 35/06, 02.20.2000.

4. Patent RU No. 2248643, IPC H01J 35/02, 03.20.2005.

5. Patent RU No. 2153848, IPC АВВ 6/00, H05G 1/20, 08/10/2000.

Claims (2)

1. An x-ray tube containing a vacuum housing, an anode and a high-voltage input with an explosion-emission cathode fixed to an insulator, characterized in that the explosion-emission cathode is made of a material with a nanostructure of the surface, which self-reproduces when exposed to high-voltage pulses with an amplitude of up to 60 kV and a duration of 10-80 ns filed by a group with a repetition rate of up to 1 MHz. 2. The x-ray tube according to claim 1, characterized in that the explosive emission cathode is made of graphite-based material.

Images